target: add async algorithm entries to the target type

On supported targets, this may be used to start a long running algorithm in
the background so the target may be interacted with during execution and
later wait for its completion.

The most obvious use case is a double buffered flash algorithm that can
upload the next block of data while the algorithm is flashing the current.

Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>